1. Field of the Invention
The present invention relates to oxidation resisting alloy steel for use typically as a catalytic substrate in an exhaust gas purifying catalytic converter, and more particularly, to an Fe-Cr-Al alloy having excellent resistance at high temperatures and useful as a catalytic substrate for such a catalytic converter.
2. Related Art
Exhaust gas purifying catalytic converters are used in the automotive and other fields to convert harmful gases such as NOx, HC and CO generated at the time of mixing and burning fuel and air to harmless products. Since the catalytic reactions tend to be exothermic, the temperature of the converter increases with use. Furthermore, recently, in many cases the converter has been located adjacent to a combustion-supporting atmosphere and the catalytic reaction has taken place in the high temperature exhaust gas. Since the converter is exposed to a high temperature atmosphere and rapid heating and cooling are repeated, the converter is subjected to excessive thermal shocks.
For use under very severe conditions, a metallic material is used such as an Fe-Cr-Al alloy having excellent oxidation resistance. Ceramics cannot normally be used due to insufficient thermal shock resistance.
Furthermore, some metallic substrates can be more easily fabricated into thin foil and fine honeycomb configurations to provide greater surface area and lighter weight as compared to ceramics.
Fe-Cr-Al alloys have been disclosed in Japanese Patent Laid-Open Nos. 48-41918 and 58-177437, Japanese Patent Publication Nos. 2-58340 and 62-14626, and Japanese Patent Laid-Open Nos. 64-30653, 1-115455 and 2-303605.
However, the materials disclosed give rise to many problems, including the following:
The material disclosed in Japanese Patent Laid-Open No. 48-41918 contains both La and Zr and has B added thereto. Furthermore, Ti must be added in order to prevent the grain size from becoming coarse. According to the present invention, B has an excellent effect on improving high temperature brittleness resistance and accordingly must be present in a proper quantity as a material for a metal honeycomb for a catalytic converter, which must also have good breakage resistance. However, since Ti has a strong effect in annulling the high temperature brittleness resistance of B even if the quantity of Ti is limited, a honeycomb foil containing Ti is brittle at high temperature. Therefore, if Ti is employed to make a metal honeycomb for a catalytic converter to be located adjacent to a hot engine, the honeycomb is vulnerable to breakage due to severe thermal shock. Accordingly, the fact that Ti is present in the material disclosed in Japanese Patent Laid-Open No. 48-41918 causes a critical defect.
The materials respectively disclosed in Japanese Patent Laid Open No. 58-177437 and Japanese Patent Publication No. 2-58340 contain La and Zr in the composition. However, the content of La is disclosed as 0.05 wt. % or less. Sufficient oxidation resistance cannot be obtained, and high temperature brittleness suffers also because B is not present.
Also, since the materials disclosed in Japanese Patent Publication No. 62-14626 and Japanese Patent Laid-Open Nos. 64-30653, 1-115455 and 2-303605 do not contain B, high temperature brittleness occurs.
Therefore, if any one of the aforesaid materials is formed into foil having a thickness of 0.2 mm or less so as to be employed for making a honeycomb for a catalytic converter, satisfactory oxidation resistance cannot be obtained, causing breakaway oxidation to take place in a short time. What is even worse, the foil for making the honeycomb is susceptible to high temperature brittleness, causing breakage of the metal honeycomb.
Typically, a 0.2 mm or less thick foil is described because it meets the commercial need for a thin foil serving as a catalytic substrate for treating an exhaust gas from an automobile engine in order to reduce the exhaust gas resistance.
As described above, conventional Fe-Cr-Al alloys have insufficient oxidation resistance and unsatisfactory durability against breakage of the honeycomb when it is employed as a material for a catalytic converter which is used at high temperature, and is formed into an alloy foil having a thickness of 0.2 mm or less. This results in breakaway oxidation in which the overall material is undesirably converted into an oxidation product or the honeycomb foil exhibits brittleness at high temperatures and is easily broken.